It is known that electrical surges in devices can have a multitude of causes.
The energy content associated with the respective overvoltage event varies greatly. It must generally be assumed, however, that overvoltage events with high energy contents are rarer than overvoltage events with low energy contents.
For example, overvoltage events with low energy contents, such as in the case of excess voltage due to switching actions, occur with far greater frequency than overvoltage events with high energy contents, such as the direct or indirect effects of lightning.
In order to render these overvoltage events less hazardous, overvoltage protection devices have been developed that are designed to divert the respective voltage surges.
However, the performance of the overvoltage protection devices also requires commensurate use of materials, so particularly effective overvoltage protection devices also come at substantial cost.
Type-I overvoltage protection devices (according to DIN EN 61643-11; previously called B-arresters according to DIN VDE 0675 part 6) are supposed to be used when high lightning currents may be coupled in.
By using type-I overvoltage protection devices, potential equalization can be established between the PE outer conductor and the neutral conductor at the time of the lightning strike. These type-I overvoltage protection devices are used in main power supply systems. This is intended to ensure that the lightning current is not able to flow into the building installation. Type-I overvoltage protection devices are supposed to operate below the rated impulse voltage of 6 kV permitted for the equipment in the feed (DIN VDE 0110 part 1/November 2003).
Type-I overvoltage protection devices generally cannot protect the entire low-voltage installation along with the terminal equipment, since the terminal equipment can be far removed and have a lower rated impulse voltage. This task is performed by overvoltage protection devices of type II (type-II overvoltage protection device according to DIN EN 61643-11; previously C-arrester according to DIN VDE 0675 part 6) and type III (type-III overvoltage protection device according to DIN EN 61643-11; previously D-arrester according to DIN VDE 0675 part 6).
Since type-I overvoltage protection devices are very expensive, a trend has developed in locations without lightning exposure to dispense with expensive type-I arresters in favor of substantially cheaper type-II arresters.
Type-II arresters are made primarily on the basis of high-performance “B40” varistor ceramic discs (edge length approx. 40 mm×40 mm). These have a rated discharge capacity Irated of about 20 kA of the 8/20-μs pulse form. Substantially higher loads result in the destruction of the arrestors.
However, the limited overvoltage protection on type-II arresters has the disadvantage that direct or nearby strikes result in pulse currents that far exceed the capacity of the type-II arrester both in terms of peak current amplitude and pulse length, resulting in its destruction.
While type-II arresters are equipped with safety mechanisms against excess heating and aging, the pulse-like overloading (of a few milliseconds) often leads to the complete destruction of the arrester.
The cause for this is that, while the corresponding backup fuse(s) are tripped in the case of larger pulse currents and thus prevent subsequent line currents from passing through overloaded arresters, the pulse current itself is not stopped, so the arrester can be overloaded without restriction.
Furthermore, the safety mechanisms of the arrester are essentially based on heat-activated mechanisms which, due to their own thermal inertia, are not tripped until after at least 100 ms.
There is consequently no effective protection against overvoltage events of excessive amplitudes and longer duration, such as long-wave pulses as a consequence of distant strikes.
Besides the pulse-like destruction of the conventional type-II arrester, this also results in direct damage in the proximity of the arrester in question in the form of mechanical destruction, metal vapor, and soot-like contamination, as well as secondary damage resulting from open electric arcs and aftereffects thereof, such as the igniting of materials that are within range.